rtwn: don't set the RTS/CTS primary channel field for RTL8812AU/RTL8821AU

According to the rtl8812au reference driver, this seems to control
the bandwidth used by lower-bandwidth frames when transmitted in
a higher bandwidth channel. For example, transmitting a 20MHz frame
on an 80MHz channel (eg in hostap mode) is doable, but you may want
to at least duplicate the RTS/CTS exchange across all four 20MHz
subchannels, AND perhaps duplicate the 20MHz frame.

I haven't fired this up with a spectrum analyser to see what the
result is.

The vendor driver doesn't bother with this and it doesn't change
performance. My guess is that for modes like AP mode we MAY wantto
be able to control the RTS/CTS bandwidth choices rather than letting
the firmare do it, but we're not there yet.

The rtl8812au code in hal/rtl8812a_xmit.c:SCMapping_8812() has
the gory details, but then the one place it's used just has it
commented out and 0 (ie "do not care") is always programmed in.

Differential Revision: https://reviews.freebsd.org/D48113
Obtained from: https://github.com/lwfinger/rtl8812au
Reviewed by: bz